This invention relates to linear drive devices for nuclear reactors and more particularly to a drive device and method of operation for increasing reliability to scram control elements into the core of the nuclear reactor.
A generally recognized method of controlling a nuclear reactor involves the use of a plurality of longitudinally movable control elements. The control elements are made of or contain a neutron absorbing material and act to regulate and control the nuclear chain reaction within the core depending on the longitudinal position of the control element relative to the core. Changes in the longitudinal position of the control elements are generally effected by linear drive devices mounted on the reactor vessel head and coupled to drive extensions of the control elements. For safety considerations, these linear drive devices must be capable of rapidly inserting (scramming) the control elements into the core to stop the nuclear chain reaction in the event of an emergency situation to prevent possible concomitant damage to the reactor and surroundings.
In the past this rapid insertion of the control elements has been accomplished by providing means associated with the linear drive device for releasing the control elements to fall freely under the influence of gravity into the nuclear core to shutdown the reactor. In particular, with magnetic jacking type drives having gripper assemblies for engaging the control element drive extension and electromagnetic coils for actuating and de-actuating the gripper assemblies, this release to scram has been accomplished by de-energizing the magnetic coils to effect disengagement of the gripper assemblies from the control element extension. Such devices have proven highly reliable in effecting a scram of the nuclear reactor.
However, the Atomic Energy Commission in a report released in September, 1973, entitled "Technical Report On Anticipated Transients Without Scram For Water-Cooled Power Reactors" and placed in the AEC's public document room, has recently stated that it would be appropriate for the nuclear industry to take reasonable steps to upgrade these reliable systems as the nuclear industry matures. The main area of concern revolves around common mode failure to scram. Common mode failure problems arise when several items are built mechanically and operate mechanically in the same way. With such devices, there is a probability, although slight, that all identical devices can fail upon the occurrence of a single event. More specifically with reference to failure to scram, there is concern that all the control element linear drive devices, such as magnetic jacks, will not function properly in the event of a scram situation to release the control elements. One suggestion for upgrading the reliability is to provide two separate shutdown or scram systems, each of which operates independently and includes different equipment from the other.
While providing two separate shutdown systems would decrease the probability of failure to scram as a result of anticipated transient (anticipated transients relate to events which are expected to occur at least once during the life of the reactor and therefore are not considered to be hypothetical accident situations), it would also significantly increase the cost of providing a nuclear power generating system. For example, it would necessarily mean increased costs in development and manufacture of a new and independent design. Furthermore, with a control element drive system using two different control element drives, it would be difficult to vary the control element management scheme so as to follow changes in fuel management.